1. Field of the Invention
The invention relates in general to a display apparatus, and more particularly to a display apparatus with a time domain multiplex driving circuit.
2. Description of the Related Art
Featuring the favorable properties of thinness, lightness and generating low radiation, liquid crystal display (LCDs) have been widely used in computer systems. A LCD panel typically uses an active matrix circuit for driving its pixels. In order to achieve a higher resolution and aperture ratio of the panel products, the industry focuses on developing improved driving circuits and associated driving methods, as well as reducing both manufacturing costs and size of the driving circuit apparatus.
FIG. 1 shows a circuit diagram illustrating a conventional LCD panel. The display panel includes a plurality of pixels (P) which are arranged in the form of a matrix on the display panel, and an active matrix driving circuit for driving the pixels. The active matrix driving circuit includes a plurality of scan lines (S), a plurality of data lines (D), and a plurality of switching devices. The switching devices are set in the pixels for selectively delivering the corresponding data signals to the pixels. Each scan line is perpendicular to each data line. Each pixel in the same pixel row is coupled to the same scan line and each pixel in the same pixel column is coupled to the same data line. The switching device can be a thin film transistor (TFT) such as an n-type field effect transistor (n-FET) or a p-type field effect transistor (p-FET). In FIG. 1, the switching device of each pixel includes at least a thin film transistor. The thin film transistor in each pixel includes a gate electrode, a first-source/drain electrode, and a second source/drain electrode. The gate electrode of the thin film transistor is coupled to the corresponding scan line and the first source/drain electrode is coupled to the corresponding data line. FIGS. 2A and 2B are the downward andsectional views of the thin film transistor structure, respectively. All electrodes of the thin film transistors are manufactured by metal or alloy, as shown by the slash line in FIG. 2B, in manufacturing the panel plate. The gate electrode is formed before the first and second source/drain electrodes are formed on the substrate when manufacturing the panel plate. Thus the gate electrode is called metal layer 1 and the first and second source/drain electrodes are called metal layer 2. Take the pixel P(m,n) for example. Suppose the pixel P(m,n) includes a thin film transistor M1 whose gate, first source/drain, and second source/drain electrodes are coupled to scan line Sm, data line Dn, and pixel capacitor C1 respectively, as shown in FIG. 1. The data lines are driven by the data drivers and the scan lines are driven by the scan drivers. Both the data driver and the scan driver are installed out of the panel. The scan drivers are used for enabling the scan lines through applying scan signals to the corresponding scan lines. When one of the scan lines is enabled, each pixel in the pixel row coupled to the enabled scan line can be turned ON. The data drivers are used for applying the data signals to the corresponding pixels through the corresponding data lines when the pixels are turned ON.
The conventional active matrix liquid crystal display has the following disadvantages. First, a large number of data lines are needed. For example, an active matrix color display panel has a resolution of 1024×768, that is, having 1024 pixel columns and having 1024×3=3072 sub-pixels for each pixel row. To drive the 3072 sub-pixels for each pixel row, the active matrix display panel requires 3072 data lines. Since a large number of the data lines are required, the pitch between the adjacent data lines must be small. Secondly, each data line is coupled to the corresponding data driver through the outer lead of the tape carrier package. Connecting all data lines to the corresponding outer leads of the tape carrier packages thus becomes difficult. Thirdly, the aperture ratio of the display panel will be decreased since the number of the data lines is so large.
FIG. 3 shows the diagram of the conventional time domain multiplex driving circuit. In the conventional time domain multiplex driving circuit, every two adjacent pixels in the same pixel row are coupled to the same data line. These two pixels are set on the left and right sides of the data line, respectively. The pixel set on the left side of the data line is called the left pixel (LP) and the pixel set on the right side of the data line is called the right pixel (RP). The switching devices of the pixels LP and RP are different. Take the pixels LP(m,n) and RP(m,n) for example. These two pixels are coupled to both the same scan line Sm and the same data line Dn. The pixel LP(m,n) is set on the left side of the data line Dn and the pixel RP(m,n) is set on the right side of the data line Dn, as shown in FIG. 3. The switching device of the pixel RP(m,n) includes a thin film transistor M2. The gate electrode of the thin film transistor M2 is coupled to the scan line Sm and the first source/drain electrode of the thin film transistor M2 is coupled to the data line Dn. The switching devices of the pixel LP(m,n) and the pixel RP(m,n) have respective configurations. The switching device of the pixel LP(m,n) includes two thin film transistors M11 and M12. The gate electrode of the thin film transistor M11 is coupled to the scan line Sm+1 while the first source/drain electrode of the thin film transistor M11 is coupled to the data line Dn. The gate electrode of the thin film transistor M12 is coupled to the scan line Sm and the first source/drain electrode of the thin film transistor M12 is coupled to the second source/drain electrode of the thin film transistor M11, as shown in FIG. 3.
FIG. 4 shows a timing chart of the respective scan signals applied to the scan lines Sm, Sm+1, and Sm+2 and the ON and OFF status of the corresponding pixels LP(m,n), RP(m,n), LP(m+1,n), and RP(m+1,n) shown in FIG. 3. The method for driving display panel with the above-described time domain multiplex driving circuit is called a time domain multiplex driving method. When the time domain multiplex driving method is executed, each pixel row is driven in turn by the time domain multiplex driving circuit. The time domain multiplex driving method includes two scanning procedures. The first scanning procedure is to selectively turn on the left pixels of the pixel row by turning on two corresponding TFTs of each of the left pixels and then feeding the corresponding data signals into the respective left pixels. The second scanning procedure is to selectively turn on the right pixels of the pixel row by turning on one corresponding TFT of each right pixel and then feeding the corresponding data signals into the respective right pixels.
Take pixels LP(m,n) and RP(m,n) shown in FIG. 3 for example. In the time period T1, the scan lines Sm and Sm+1 are enabled so that the thin film transistors M11 and M12 can be turned ON and a data signal can be applied to the corresponding pixel LP(m,n) through the TFTs M11 and M12. In the time period T2, only the scan line Sm is enabled. The thin film transistor M2 can be turned ON and a data signal can be applied to the corresponding pixel RP(m,n) through the TFT M2.
In the time domain multiplex driving circuit, the above-described disadvantages of the conventional active matrix driving circuit can be improved. If the resolution of the display panel is 1024×768, for example, every two adjacent pixels in the same pixel row are coupled to one corresponding data line of the time domain multiplex driving circuit, and thus only 3072/2=1536 data lines are needed.
However, the conventional time domain multiplex driving circuit described above has the following disadvantage. First, a longer scanning time for pixels is needed. When the TFT is turned ON, an equivalent output resistor RO between the first and second source/drain electrodes is produced. The equivalent output resistor RO can affect scanning time needed when the pixel rows are being scanned. The larger the equivalent output resistor RO is, the longer the time needed to perform scanning will be. In other words, the scanning rate will be slower. Take the pixels LP(m,n) and RP(m,n) shown in FIG. 3 for example. The switching device of the pixel LP(m,n) includes two serially connected TFTs M11 and M12. When the mth pixel row is scanned, the TFTs M11 and M12 are enabled, resulting in a resistance equivalent to the resistance of the respective output resistors of the TFTs M11 and M12 in series. Therefore, LP(m,n) has an equivalent output resistance of 2RO, that is, two times larger than the equivalent output resistance of the conventional switching device structure shown in FIG. 1. Therefore, when the pixels are driven by the time domain multiplex driving circuit, the scanning time needed to apply all data signals to the corresponding pixels must be longer.
Secondly, the luminance uniformity of the display cannot be achieved due to feed-through effect. Referring to FIG. 2, the coverage areas of the gate electrode (G) and the second source/drain electrode (S/D-2) on the panel overlap each other, which can be seen when TFTs on the panel are being downward. The overlapping areas between the gate electrode (G) and the second source/drain electrode (S/D-2) are substantially equivalent to a feed-through capacitor CFT 202. The output voltage of the TFT is lower than the input voltage of the TFT and the luminance of the pixel is degraded because of the equivalent feed-through capacitor 202. This phenomenon is called feed-through effect. The difference between the input voltage and the output voltage is called feed-through voltage. The larger the capacitance of the equivalent capacitor is, the larger the feed-through voltage is. Take the pixels LP(m,n) and RP(m,n) shown in FIG. 3 for example. The switching device of the pixel RP(m,n) includes only one TFT M2 and the switching device of the pixel LP(m,n) includes two TFTs M11 and M12. The data signal applied to the pixel RP(m,n) only through the TFTs M2 but the data signal applied to the pixel LP(m,n) through two TFTs, M11 and M12. Therefore, the equivalent capacitor of LP(m,n) is much larger than that of RP(m,n). During the driving of the pixels by the time domain multiplex driving circuit, the pixel LP(m,n) will have smaller luminance than that of the pixel RP(m,n) if the data signals of equal magnitude are applied to the pixel LP(m,n) and RP(m,n) respectively. Therefore, the luminance of the adjacent pixels may not be the same even when the data signals of equal magnitude are applied to the pixels respectively. The display performance of the liquid crystal display would thus be degraded.
In addition, the luminance of a display panel whose pixels are arranged according to the structure shown in FIG. 3 would be non-uniform when identical data signals are applied to all pixels of the display. This phenomenon can be referred to as odd-even line effect. For the display panel according to FIG. 3, each pixel of the odd (or even) pixel columns includes two TFTs and each pixel of the even (or odd) pixel columns includes one TFT, so that the equivalent capacitances of the adjacent pixel columns are different, thus resulting in the non-uniformity of luminance. The display quality of the liquid crystal display may be degraded because of the odd-even line effect.
According to the above descriptions, the conventional time domain multiplex driving circuit has the following disadvantages. First, the scanning time needed to activate pixels is longer. Secondly, the luminance of the display is not uniformly over the whole panel. Thirdly, the odd-even line effect degrades the display quality.